Trench process and structure for backside contact solar cells with polysilicon doped regions

ABSTRACT

A solar cell includes polysilicon P-type and N-type doped regions on a backside of a substrate, such as a silicon wafer. A trench structure separates the P-type doped region from the N-type doped region. Each of the P-type and N-type doped regions may be formed over a thin dielectric layer. The trench structure may include a textured surface for increased solar radiation collection. Among other advantages, the resulting structure increases efficiency by providing isolation between adjacent P-type and N-type doped regions, thereby preventing recombination in a space charge region where the doped regions would have touched.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/879,847, filed Sep. 10, 2010, which is a divisional of U.S.application Ser. No. 12/431,684, filed Apr. 28, 2009, which claims thebenefit of U.S. Provisional Application No. 61/060,921, filed Jun. 12,2008, all of which are incorporated herein by reference in theirentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to solar cells, and moreparticularly but not exclusively to solar cell fabrication processes andstructures.

2. Description of the Background Art

Solar cells are well known devices for converting solar radiation toelectrical energy. They may be fabricated on a semiconductor wafer usingsemiconductor processing technology. A solar cell includes P-type andN-type doped regions. Solar radiation impinging on the solar cellcreates electrons and holes that migrate to the doped regions, therebycreating voltage differentials between the doped regions. In a backsidecontact solar cell, both the doped regions and the interdigitated metalcontact fingers coupled to them are on the backside of the solar cell.The contact fingers allow an external electrical circuit to be coupledto and be powered by the solar cell.

Efficiency is an important characteristic of a solar cell as it isdirectly related to the solar cell's capability to generate power.Accordingly, techniques for increasing the efficiency of solar cells aregenerally desirable. The present invention allows for increased solarcell efficiency by providing processes for fabricating novel solar cellstructures.

SUMMARY

In one embodiment, a solar cell includes polysilicon P-type and N-typedoped regions on a backside of a substrate, such as a silicon wafer. Atrench structure separates the P-type doped region from the N-type dopedregion. Each of the P-type and N-type doped regions may be formed over athin dielectric layer. The trench structure may include a texturedsurface for increased solar radiation collection. Among otheradvantages, the resulting structure increases efficiency by providingisolation between adjacent P-type and N-type doped regions, therebypreventing recombination in a space charge region where the dopedregions would have touched.

These and other features of the present invention will be readilyapparent to persons of ordinary skill in the art upon reading theentirety of this disclosure, which includes the accompanying drawingsand claims.

DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 show a solar cell structure in accordance with anembodiment of the present invention.

FIGS. 3, 4, 5, 6, 7A, 8A, 7B, 8B, 9 and 10 illustrate the fabrication ofa solar cell in accordance with an embodiment of the present invention.

FIG. 11 shows dark I-V curves comparing the performance of aconventional solar cell to a solar cell that is in accordance with anembodiment of the present invention.

FIG. 12 shows a flow diagram of a method of fabricating a solar cell inaccordance with an embodiment of the present invention.

The use of the same reference label in different figures indicates thesame or like components. The figures are not drawn to scale.

DETAILED DESCRIPTION

In the present disclosure, numerous specific details are provided, suchas examples of materials, process parameters, process steps, andstructures, to provide a thorough understanding of embodiments of theinvention. Persons of ordinary skill in the art will recognize, however,that the invention can be practiced without one or more of the specificdetails. In other instances, well-known details are not shown ordescribed to avoid obscuring aspects of the invention.

In solar cells with P-type and N-type doped regions in the substrate,the P-type and N-type doped regions may be formed with separate orabutting perimeters. The inventor discovered, however, that this is nottrue with polysilicon doped regions because recombination in the spacecharge region where the polysilicon doped regions touch is very high dueto the lifetime of charge carriers in the polysilicon being very low.That is, the inventor discovered that touching polysilicon doped regionsadversely affect efficiency. Embodiments of the present inventionaddress this problem associated with polysilicon doped regions andformed doped regions in general.

FIG. 1 schematically shows a sectional view of a solar cell structure inaccordance with an embodiment of the present invention. In the exampleof FIG. 1, the solar cell is a backside contact solar cell in that itsdoped regions 101 and 102 are on the backside 106 opposite to the frontside 105. The front side 105 faces the sun during normal operation. Thedoped regions 101 and 102 are formed on a thin dielectric layer 113. Thedielectric layer 113 may be formed to a thickness of 5 Angstroms to 40Angstroms. In one embodiment, the dielectric layer 113 comprises silicondioxide thermally grown on the surface of the substrate 103 to athickness of 20 Angstroms. The dielectric layer 113 may also comprisesilicon nitride. The dielectric layer 113 advantageously allows forsurface passivation. The polysilicon of the doped regions 101 and 102applies an electric field across the dielectric layer 113, which repelsminority carriers and accumulates majority carriers at the dielectricinterface.

In the example of FIG. 1, the doped region 101 is a P-type doped region,while the doped region 102 is an N-type doped region. A substrate 103comprises an N-type silicon wafer in this example. As can beappreciated, the substrate 103 may also comprise a P-type silicon orother wafer with appropriate changes to the rest of the structure. Thereare several P-type and N-type doped regions in any given solar cell butonly one of each is shown in FIG. 1 for clarity of illustration.

The doped regions 101 and 102 may comprise doped polysilicon formed to athickness of about 2000 Angstroms by low pressure chemical vapordeposition (LPCVD). The doped region 101 may comprise polysilicon dopedwith a P-type dopant (e.g., boron) and the doped region 102 may comprisepolysilicon doped with an N-type dopant (e.g., phosphorus). Thepolysilicon may be deposited over the thin dielectric layer 113 and thendoped by diffusion. The polysilicon may also be pre-doped prior todeposition on the dielectric layer 113. Polysilicon is the preferredmaterial for the doped regions 101 and 102 for its compatibility withhigh temperature processing, allowing for increased thermal budget.

As shown in FIG. 1, the doped regions 101 and 102 are separated by atrench 104, which serves as a gap between the doped regions 101 and 102.

The trench 104 may be formed by laser trenching or conventional etching,for example. In one embodiment, the trench 104 is about 100 micronswide. The trench 104 may be formed before or after a diffusion step thatdopes the polysilicon doped regions 101 and 102. If the trench 104 isformed before the diffusion step, the passivation region 112 maycomprise an N-type passivation region formed during the diffusion step.

In one embodiment, the trench 104 is formed using a process that notonly forms the trench 104 but also forms a randomly textured surface 114on the surface of the trench 104. The randomly textured surface 114improves solar radiation collection of light incident on the back of thesolar cell, i.e. a bifacial configuration. A wet etch process comprisingpotassium hydroxide and isopropyl alcohol may be used to form the trench104 and to texture the surface 114 with random pyramids. The trench 104may be formed to dig 1 to 10 microns (e.g., 3 microns) into thesubstrate 103.

A dielectric in the form of a silicon nitride 107 is deposited in thetrench 104. The silicon nitride 107 preferably has a relatively largepositive fixed charge density to place the silicon surface under thetrench 104 in accumulation and to provide good surface passivation. Thepositive fixed charge density of the silicon nitride 107 may naturallyoccur as part of the deposition process used to form the silicon nitride107. In one embodiment, the silicon nitride 107 is formed to a thicknessof about 400 Angstroms by plasma enhanced chemical vapor deposition(PECVD). The resulting accumulation layer repels minority carriers, i.e.positively charged holes in N-type material. The trench 104 alsoprevents the space charge region from developing in the polysilicon.Instead, the space charge develops in the single crystal siliconunderneath the P-type polysilicon. In this region, lifetime is notreduced due to grain boundaries, and hence the parasitic recombinationis suppressed. A portion of this space charge region also intersects thesurface of the wafer in the trench 104. The positive charge in thesilicon nitride 107 reduces the impact of this region of space chargeregion as well narrowing the region.

An example process flow for fabricating the solar cell structure of FIG.1 may include forming a thin dielectric layer 113 over a backsidesurface of the substrate 103, forming an undoped polysilicon layer overthe thin dielectric layer 113, doping the polysilicon layer into P-typeand N-type doped regions 101 and 102, etching the doped polysiliconlayer to form the trench 104 and the textured surface 114, forming thepassivation region 112, and forming the silicon nitride 107 in thetrench 104. Rather than diffusing dopants on an undoped polysiliconlayer, the doped regions 101 and 102 may also be formed by depositingpre-doped polysilicon on the dielectric layer 113 using conventionaldeposition, masking, and etching techniques. The silicon nitride 107preferably has a planar, as opposed to textured, surface. However, theplanarity of the silicon nitride 107 is not critical and no additionalplanarization step is needed. For example, the planarity of the siliconnitride 107 may be as deposited. The trench 104 may be formed before orafter doping of the doped regions 101 and 102.

Referring to FIG. 2, interdigitated metal contact fingers 108 and 109may be formed through the silicon nitride 107 to make an electricalconnection to the doped regions 101 and 102, respectively. Externalelectrical circuits may be attached to the interdigitated metal contactfingers 108 and 109 to connect to and be powered by the solar cell. Inthe example of FIG. 2, the metal contact finger 108 may be connected toa positive electrical terminal and the metal contact finger 109 may beconnected to a negative electrical terminal.

The trench structure of FIG. 1 addresses the aforementioned issuesrelating to polysilicon parasitic space charge recombination severalways. Firstly, the trench 104 separates the doped regions 101 and 102 sothey are not physically in contact. This prevents the space chargeregion from existing in either polysilicon film. Secondly, the resultingaccumulation layer under the trench 104 repels minority carriers toimprove surface passivation. Thirdly, the textured surface 114 in thetrench 104 increases solar radiation collection. These advantageouslyhelp increase solar cell efficiency.

FIGS. 3-10 show sectional views illustrating the fabrication of a solarcell in accordance with an embodiment of the present invention. Thereare a plurality of P-type doped regions and N-type doped regions in asolar cell but only one of each is shown as being fabricated in thefollowing example for clarity of illustration.

The embodiment of FIGS. 3-10 begins with formation of a thin dielectriclayer 313 on a backside surface of a substrate 303 (FIG. 3). Thesubstrate 303 may comprise an N-type silicon wafer, for example. Thedielectric layer 313 may be formed to a thickness of 5 Angstroms to 40Angstroms (e.g., 20 Angstroms). In one embodiment, the dielectric layer313 comprises silicon dioxide thermally grown on the surface of thesubstrate 103. The dielectric layer 313 may also comprise siliconnitride, for example. An undoped polysilicon layer 322 is then formed onthe dielectric layer 313. The polysilicon layer 322 may be formed to athickness of about 2000 Angstroms by LPCVD, for example. A doped silicondioxide layer 323 is then formed over the polysilicon layer 322 (FIG.4). The silicon dioxide layer 323 serves as a dopant source for asubsequently formed doped region, which is a P-type doped region 301 inthis example (see FIG. 7A or 8B). The silicon dioxide layer 323 may thusbe doped with a P-type dopant, such as boron. The doped silicon dioxidelayer 323 is patterned to remain over an area of the polysilicon layer322 where the P-type doped region 301 is to be formed (FIG. 5). Thesilicon dioxide layer 323 may be formed to a thickness of about 1000Angstroms by APCVD.

A doped silicon dioxide layer 324 is formed over the silicon dioxide 323and the polysilicon layer 322 (FIG. 6). The silicon dioxide 324 servesas a dopant source for a subsequently formed doped region, which is anN-type doped region 302 in this example (see FIG. 7A or 8B). The silicondioxide 324 may thus be doped with an N-type dopant, such as phosphorus.The silicon dioxide 324 may be formed to a thickness of about 2000Angstroms by APCVD.

The trench separating the doped regions may be formed before formationof the doped regions in a first trench formation process or afterformation of the doped regions in a second trench formation process.FIGS. 7A and 8A illustrate process steps for the first trench formationprocess, while FIGS. 7B and 8B illustrate process steps for the secondtrench formation process. Both trench formation processes may proceedfrom FIG. 6 and continue on to FIG. 9.

In the first trench formation process, a thermal drive-in step diffusesdopants from the silicon dioxides 323 and 324 to the underlyingpolysilicon layer 322, thereby forming P-type and N-type doped regionsin the polysilicon layer 322, which is accordingly relabeled as P-typedoped region 301 and N-type doped region 302 (FIG. 7A). The thermaldrive-in step may be performed by heating the sample of FIG. 6. Thepreferred drive conditions give a heavily doped, e.g., greater than 1e²⁰cm⁻³, polysilicon layer that is uniform throughout the thickness ofthe film and has very little doping under the polysilicon, e.g., equalto or less than 1 e¹⁸cm⁻³. The thermal drive-in step results in thepolysilicon layer 322 under the silicon dioxide 323 forming the P-typedoped region 301 and polysilicon layer 322 under the silicon dioxide 324forming the N-type doped region 302.

The silicon dioxide 324, silicon dioxide 323, doped region 301, dopedregion 302, and thin dielectric layer 313 are etched to form a trench304 (FIG. 8A). The trench etch may comprise a multi-step etch process,with the last etch step stopping on the substrate 303. The trench 304may be about 100 microns wide, for example. However, there is no knownlimit to the minimum width as long as the P-type doped region 301 andN-type doped region 302 do not contact each other. The trench 304 may beformed by conventional etching processes including by laser trenching.In one embodiment, the trench 304 has a textured surface 314 forimproved solar radiation collection efficiency. In one embodiment, a wetetch process comprising potassium hydroxide and isopropyl alcohol isused to form the trench 304 and to texture the surface 314 with randompyramids. The trench 304 may extend 1 to 10 microns, e.g., 3 microns,into the substrate 303.

A thin (less than 200 Angstroms, e.g., 100 Angstroms) passivation layer310 may be formed on the surface 314 of the trench 304. The passivationlayer 310 may comprise silicon dioxide thermally grown on the surface314 or deposited silicon nitride layer, for example.

In the second trench formation process, the silicon dioxide 324, silicondioxide 322, and thin dielectric layer 313 of the sample of FIG. 6 areetched to form the trench 304 (FIG. 7B). Textured surface 314 may beformed on the surface of the trench 304. The trench etch is essentiallythe same as in the first trench formation process except that the trenchis formed before formation of the doped regions of the solar cell.

A thermal drive-in step is performed to diffuse dopants from the silicondioxide layers 323 and 324 to the underlying polysilicon layer 322,thereby forming the doped regions 301 and 302 as in the first trenchformation process (FIG. 8B). In this case, in the second trenchformation process, a passivation region 315 is formed in the substrate303 under the trench 304 during the diffusion process. The passivationregion 315 may comprise diffused N-type dopants. In one embodiment, thepassivation region 315 is formed by introducing POCl3 (phosphoruschloride oxide) in the diffusion furnace during the thermal drive-in.The passivation region 315 serves the same function as the passivationregion 112 of FIG. 1.

In both the first and second trench formation processes, the trench 304serves as a gap physically separating the P-type doped region 301 fromthe N-type doped region 302. The processing of the solar cell continuesfrom either FIG. 8A or 8B to FIG. 9.

Continuing with FIG. 9, a dielectric in the form of a silicon nitridelayer 307 is formed in the trench 304. In the example of FIG. 9, thesilicon nitride layer 307 is also formed over the layers 323 and 324.The silicon nitride layer 307 preferably has a relatively large positivefixed charge density to place the silicon surface under the trench 304in accumulation and to provide good surface passivation. The positivefixed charge density on the silicon nitride layer 307 may naturallyoccur as part of a PECVD process, for example. In one embodiment, thesilicon nitride 307 is formed to a thickness of about 400 Angstroms byPECVD. The silicon nitride 307 preferably has a planar (e.g., asdeposited) surface. In FIGS. 9 and 10, the passivation region 312represents either the passivation layer 310 (see FIG. 8A) or thepassivation region 315 (see FIG. 8B) depending on the trench formationprocess used.

Interdigitated metal contact fingers 308 and 309 may then be formedthrough the silicon nitride 307 to make an electrical connection to thedoped regions 301 and 302 by way of layers 323 and 324, respectively(FIG. 10). External electrical circuits may be attached to theinterdigitated metal contact fingers 308 and 309 to connect to and bepowered by the solar cell. In the example of FIG. 10, the metal contactfinger 308 may be coupled to a positive electrical terminal and themetal contact finger 309 may be coupled to a negative electricalterminal. The resulting solar cell provides the same benefits as thesolar cell of FIG. 1.

FIG. 11 shows dark I-V (i.e., current-voltage) curves comparing theperformance of a conventional solar cell to a solar cell that is inaccordance with an embodiment of the present invention. The I-V curvesare “dark” in that they were measured with no direct solar radiationshining on the solar cells.

The I-V curves are for the diodes formed between an N-type silicon and aP-type doped region. In the example of FIG. 11, the horizontal axisrepresents voltage across the diode and the vertical axis represents theresulting current across the diode. Plot 401 is the I-V curve for aconventional solar cell with touching P-type and N-type polysilicondoped regions, plot 402 is the I-V curve for a typical SunpowerCorporation A300™ solar cell, and plot 403 is for a solar cell having atrench between the P-type and N-type doped regions as in FIGS. 1 and 9.While the plot 402 is very close to the ideal I-V curve represented bythe plot 404, the plot 403 is even closer. The plot 405 represents aguide for the eye of an ideal diode I-V characteristic, the slope ofwhich is 60 millivolts per decade of current.

Referring now to FIG. 12, there is shown a flow diagram of a method 600of fabricating a solar cell in accordance with an embodiment of thepresent invention. In the method 600, doped regions are formed in apolysilicon layer (step 601). The doped regions may be formed bydepositing doped silicon dioxide layers over an undoped polysiliconlayer and performing a diffusion step, by depositing pre-doped silicondioxide layers, or by depositing an undoped polysilicon layer followedby a dopant implantation step, for example. The polysilicon layer wherethe doped regions are formed may be etched to form a trench separatingthe P-type doped region from the N-type doped region (step 602).Alternatively, the trench is formed before the doped regions are formed.The trench may include a textured surface for increased solar radiationcollection. A passivation region, such as passivation layer or adiffused region in the substrate, may be formed to isolate trenchmaterial from the bulk of the substrate (step 603). A dielectric in theform of a silicon nitride layer may then be deposited in the trench(step 604). Interdigitated metal contact fingers may thereafter beformed to electrically connect to the P-type and N-type doped regionsthrough the silicon nitride.

Improved solar cell fabrication processes and structures have beendisclosed. While specific embodiments of the present invention have beenprovided, it is to be understood that these embodiments are forillustration purposes and not limiting. Many additional embodiments willbe apparent to persons of ordinary skill in the art reading thisdisclosure.

What is claimed is:
 1. A solar cell comprising: a solar cell substratehaving a front side configured to face the sun during normal operationand a backside opposite the front side; a P-type doped region and anN-type doped region of the solar cell over the backside of the solarcell substrate; a first dielectric layer between the solar cellsubstrate and the P-type and N-type doped regions; and a trenchstructure separating the P-type doped region and the N-type dopedregion, and dividing the first dielectric layer.
 2. The solar cell ofclaim 1 wherein the P-type and N-type doped regions comprisepolysilicon.
 3. The solar cell of claim 1 wherein the solar cellsubstrate comprises an N-type doped silicon substrate.
 4. The solar cellof claim 1 wherein the first dielectric layer comprises silicon dioxideformed to a thickness between 5 to 40 Angstroms on a backside surface ofthe solar cell substrate.
 5. The solar cell of claim 1 wherein thetrench has a textured surface that absorbs solar radiation incident onthe backside of the solar cell substrate.
 6. The solar cell of claim 1further comprising a second dielectric layer in the trench.
 7. The solarcell of claim 6 further comprising: a passivation layer between thesecond dielectric layer and the solar cell substrate.
 8. The solar cellof claim 1 further comprising a diffused passivation region in the solarcell substrate under the trench, wherein the diffused passivation regionis doped with an N-type dopant.
 9. The solar cell of claim 1 furthercomprising interdigitated metal contact fingers electrically coupled tothe P-type and N-type doped regions.
 10. A solar cell comprising: aP-type doped region and an N-type doped region formed on a backside of asolar cell substrate, each of the P-type and N-type doped regions beingformed over a first dielectric layer; and a trench structure separatingthe P-type doped region and the N-type doped region, and dividing thefirst dielectric layer.
 11. The solar cell of claim 10 furthercomprising: a second dielectric layer formed in the trench structure.12. The solar cell of claim 11 further comprising metal contactselectrically coupled to the P-type and N-type doped regions through thesecond dielectric layer.
 13. The solar cell of claim 10 wherein thesolar cell substrate comprises an N-type silicon substrate.
 14. Thesolar cell of claim 10 wherein the P-type doped region and the N-typedoped region comprise polysilicon.
 15. The solar cell of claim 10wherein a surface of the trench structure is randomly textured.